Electromagnetic induction caused by high-frequency magnetic flux is often used to supply power in a contactless manner to cell phones, electric household appliances, and the like. Such electromagnetic induction is used not only for proximity-type electromagnetic induction power supplying that has already been put into practical use but also in spatial power supplying technology, which is referred to as magnetic resonance type power supplying and becoming popular these days. The spatial power supplying technology allows for power supplying over a certain distance.
High-frequency magnetic flux also generates induced electromotive power in a metal foreign object and increases the temperature due to eddy current loss. When the temperature of a metal foreign object becomes high, the housing of a power supplying device or appliance may be deformed. In addition, a user may inadvertently touch the metal foreign object and get hurt.
Several inventions have been proposed to prevent induction heating of such metal object (e.g., Patent Document 1 and Patent Document 2). In the proposed systems, a high-frequency inverter circuit arranged in a power supplying device intermittently oscillates in a standby state. Even when a metal foreign object is solely set during the standby period, the average output is extremely small. Thus, the temperature subtly increases, and the system is safe to use.
When the proper appliance is set, a slight amount of power is transmitted during the intermittent oscillation period to a secondary coil through a primary coil. The secondary side (the appliance side) generates a special signal with the transmitted power and sends back the special signal through an antenna arranged in the power supplying device. By detecting the signal, the power supplying device determines whether or not a proper appliance is set and controls the high-frequency inverter.
Specifically, the power supplying device performs authentication to determine whether or not the appliance is a proper one. Thus, the intermittent oscillation continues when only a metal foreign object is set, and the system is safe to use.
Patent Document 1: Japanese Patent No. 3392103
Patent Document 2: Japanese Patent No. 3306675